I'd like to point out something so incredibly obscure 99% of everyone will miss this. It comes from " the dead ball ERA " And the rule was never removed from the books, which means it's still in force today. A player will not blow the ball into foul territory to assist the fielding team on any ball hit ( tapped nowadays ) slowly down the first or third baseline. It happened once, and the people who made the rule didn't think that it was fair play, so they all passed a law outlawing the practise. And that is a "did you know.." for everyone to try to locate. But it's a really good one .

That's the intent of the rule. It's to prevent easy double plays. If the ball can be caught with normal effort, with less than 2 outs and runners on first and second or first, second and third, the ump should shout, "INFIELD FLY RULE. BATTER IS OUT.", at which point, runners can safely stay where they are. Apparently, no ump called it, so the runners had to advance.